FIG. 5 shows a conventional class B push-pull power amplifier having an output stage of a collector output type. The power amplifier comprises an output stage having a pair of power transistors Q1 and Q2, and a driver stage having a pair of driver transistors Q3 and Q4. Both of the power transistors Q1 and Q2 are connected to each other at collectors thereof and led to an output terminal OUT. The driver transistors Q3 and Q4 are provided for driving the power transistors Q1 and Q2, respectively.
In the power amplifier of the collector output type, loss at the driver transistors Q3 and Q4 becomes large compared with a power amplifier having an output stage where the output is produced through an emitter as described below. In the power amplifier of the emitter output type, an emitter-collector voltage decreases as the base current of the power transistor increases, so that the loss at the driving transistors is small. To the contrary, in the power amplifier of the collector output type, if the base current of each of the power transistors Q1 and Q2 increases, temperature of each of the driver transistors Q3 and Q4 increases. Thus, the base-emitter voltage (Vbe), of each of the driver transistors Q 3 and Q4 decreases, so that the bias current of the driver transistor increases to increase the loss at the transistor.
In order to eliminate such a disadvantage, the power amplifier is provided with a temperature compensation circuit. The circuit comprises a pair of diodes D1 and D2 connected to the collectors of the driver transistors Q3 and Q4 and a thermistor S connected to bases of the driver transistors Q3 and Q4. The diodes D1 and D2 and the thermister S are thermally coupled with the transistors Q1 and Q2 so as to receive the heats of the transistors.
In place of the thermistor S, a bias setting and temperature compensation circuit B is provided as shown in FIG. 6. The bias voltage setting and temperature compensation circuit B is a constant voltage circuit and comprises a bias setting transistor Q5, and a pair of resistors R11 and R12 connected between input voltages Vi+and Vi-`and a base of the transistor Q5. The voltage between the bases of the driver transistors Q3 and Q4 is the value obtained by multiplying the base-emitter voltage VBE of the transistor Q5 and (R11+R12)/R12. Thus, the voltage between the bases of the driver transistors Q3 and Q4 is compensated by the change of the voltage VBE of the transistor Q5 dependent on temperature change.
However, in such a circuit, the temperature of the driver transistor increases with the bias current, which causes a vicious circle of heat. Therefore, it is necessary to provide a complicated circuit for the temperature compensation.
Furthermore, in the circuit of FIG. 6, the voltage between the bases of the transistors is compensated with respect to the heat by the temperature change of V.sub.BE of the transistor Q5 at a temperature coefficient which is (R11+R12)/R12 times of as large as the temperature coefficient of the transistor Q5. In order to provide a superfluous temperature compensation, if the total of temperature coefficient of the voltage V.sub.BE of the driver transistors Q3 and Q4 is -4 mV/.degree. C., the temperature coefficient of the circuit B must be increased over the total value.